So in his mind, he watched Phillies starter Cole Hamels unload his arsenal. He prepared for success, modest success, making solid contact, extending his at-bats, aiding his team. Even then, he did not imagine “necessarily, a day like this,” he said.

With his team already threatening to sag after the All-Star break, Hairston erupted with five RBI, the highest total of his eight-season career. He doubled twice and launched a seventh-inning homer. His second-deck blast provided the crescendo for the 41,166 fans at Citi Field.

The baseball season grants few days like this, and the Mets would do well to savor this one. The coming weeks will be hard. This group understands its mandate: Win and make inroads in the playoff chase or watch as the team is disassembled. The odds are long. The Mets (47-46) already saw their closer shipped out of town in the middle of the night.

So they should draw enjoyment from this one: They won with a lineup missing the four best players on the roster. They charged a season-high seven runs to the ledger of one of the National League’s most elusive starters. They enjoyed watching their own starter, Jon Niese (9-7, 3.73 ERA), look both mature and capable during seven innings.

Before the game, manager Terry Collins learned that Beltran felt awful. A fever wracked him through the night, and he never made it to the park today. So with his lineup already missing Jose Reyes, David Wright and Ike Davis, Collins penciled Hairston’s name into the No. 3 spot.

Why not? In 18 plate appearances against Hamels, Hairston had six hits. The sample size was small, but Collins felt compelled anyway.

Last weekend in San Francisco, he launched a game-winning home run off All-Star closer Brian Wilson.
Afterward, his face cracked into a tight grin.

In the first inning, Hairston rewarded his manager. He splashed a double to right and scored on an accident. The next batter, Daniel Murphy, uppercut a fastball high above the infield. Ryan Howard and Chase Utley moved toward the baseball, then veered in the opposite direction. The grass served as a landing spot. Hairston charged home.

For Hamels, it would only get worse. In the third inning, Niese earned a seven-pitch walk and Justin Turner smacked a single. Hairston then smoked a 90-mph fastball down the third-base line past former Met Wilson Valdez. In left field, Raul Ibanez doddered around the ball, unable to field it. Both runners scored, and Hairston had his second double of the game.

From the on-deck circle, Murphy witnessed the display. “They were rockets today that he was hitting,” said Murphy, who added a solo homer in the fifth.

By the seventh inning, the game’s momentum had tilted ever so slightly. The Phillies tagged Niese for a pair of runs, and the ballpark atmosphere dipped. Hairston would awaken the crowd.

With two men on, reliever Danys Baez attempted to throw a middle-in fastball past him. No chance. The resulting blast landed in the second deck of left field, the final bow on a day that he could only imagine.